The LWF Blog
Fire Safety Engineering for Design – Types of Water Mist System – Part 301
August 24, 2026 8:50 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 300, LWF looked at the types of water mist system, particularly low pressure water mist systems. In part 301, we continue looking at types of water mist system by discussing single fluid systems operating at 70 bar or above.
Single-fluid water mist systems operating at 70 bar or above require a significantly different approach to the water supply and distribution system. At these pressures, the equipment is no longer comparable with conventional sprinkler hardware. The pump, pipework, valves, storage vessels and discharge orifices all have to be designed around sustained high-pressure operation.
Positive-displacement pumps are used because they are well suited to generating the high pressures required by these systems. Unlike a centrifugal pump, where pressure and flow are closely related to the pump curve and system resistance, a positive-displacement pump delivers a defined volume of water for each cycle or revolution. System pressure is therefore controlled through the interaction between pump output, system demand and pressure-control equipment. The pump must be selected not only for its maximum pressure but also for the flow required by the approved nozzle arrangement and the duration of operation.
The distribution pipework becomes a critical part of the pressure boundary. Stainless steel hydraulic tubing is commonly used because it provides the mechanical strength and corrosion resistance required for high-pressure water service. Connections and fittings must be compatible with the specified working pressure, while installation also needs to account for vibration, thermal movement and the mechanical consequences of a pressurised system. At 70 bar and above, seemingly minor weaknesses in fittings or joints become much more significant than they would be in conventional sprinkler pipework.
The same principle applies to valves. Specially fabricated or purpose-designed high-pressure valves are required because standard fire-protection valves are not necessarily suitable for these operating conditions. Valve bodies, seals, connections and actuation arrangements all have to withstand the system pressure while remaining capable of reliable operation when the system is activated. The pressure rating of individual components therefore has to be considered as part of the complete hydraulic pressure boundary.
Water storage introduces another engineering consideration. Cylinders with working pressures around 200 bar provide a substantial pressure margin above the operating pressure of the water system. The internal plastic lining described in the original arrangement has an important role in separating the stored water from the cylinder material, reducing the potential for corrosion and maintaining the integrity of the pressure vessel. The cylinder is consequently both a water-storage component and a high-pressure mechanical vessel, requiring appropriate inspection and maintenance.
The generation of the mist occurs at the final stage of the system. Water reaches the nozzle under high pressure and passes through finely drilled orifices. The pressure energy is converted as the water accelerates through the small opening, producing a high-velocity discharge that breaks the water into fine droplets. Orifice dimensions are therefore fundamental to the system’s performance. A small change in diameter can alter the discharge flow and the resulting spray characteristics, making nozzle manufacture and dimensional control particularly important.
This also explains why the orifice cannot be considered independently of the system pressure. The nozzle is designed to operate within a defined pressure and flow range, and the resulting spray pattern must be demonstrated through the applicable fire-testing and approval process. BS EN 14972-1:2020+A1:2025 covers the design, installation, inspection and maintenance of fixed water mist systems and limits its application to occupancies covered by the relevant EN 14972 fire-test protocols.
At these pressures, therefore, the engineering is concentrated around maintaining a reliable pressure boundary and converting that pressure into a controlled discharge. The positive-displacement pump, stainless-steel hydraulic system, high-pressure valves, lined storage cylinder and precision orifices are not separate pieces of equipment; they form the chain that allows the stored hydraulic energy to be converted into the water mist on which the fire performance depends.
In part 302 of LWF’s series on fire engineering we will continue discussing types of water mist system and consider Twin Fluid Systems. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.